1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly, to a method of manufacturing a semiconductor device, which can prevent a characteristic of a gate oxide from degrading and can prevent deformation of the profile of a recess top portion and active loss of a region around a peripheral circuit during light etch treatment in the course of forming a step-gated asymmetry recess (STAR) cell.
2. Description of the Prior Art
In accordance with rapid advance of high integration of a memory semiconductor device such as a DRAM, there are problems in that a threshold voltage margin and a refresh time of a cell region are reduced in a construction of a conventional flat transistor. Several studies have been actively performed in order to secure the refresh characteristic together with a threshold voltage satisfying the requirement for the high integration of the device.
As one example of such studies, a structure of a STAR (step-gated asymmetry recess) cell has been proposed. The STAR cell has a structure in which a portion of an active region is recessed to form a stepped portion at the active region and a gate is formed at the stepped portion of the active region, thereby extending an effective channel length in a MOSFET device. According to the structure of the STAR cell, the short channel effect is reduced, so that a desired level of the threshold voltage can be obtained by a low threshold voltage dose. Also, the electric field across the MOSFET device can be lowered. Therefore, the refresh time to update data can be improved to be three times higher than the structure of the conventional flat cell.
In particular, such a STAR cell can be achieved by adding a simple step into an existing process or simply altering the existing process. Since the STAR cell is very easily applied, it has emerged as a very effective method to solve the reduction of the threshold voltage and refresh time due to the high integration of the memory semiconductor device.
A method of manufacturing the semiconductor device to form a conventional STAR cell will now be described in brief.
First, a trench type isolation film is formed in a field region of a semiconductor device to define an active region. After an antireflective film is deposited on the entire surface of the substrate, a photosensitive film pattern is formed on the antireflective film while exposing a portion of the isolation film or the active region adjacent to the isolation film.
Then, the antireflective film, the isolation film, and the active region are etched using the photosensitive film pattern as an etching mask, so that a portion of the isolation film and the active region adjacent to the isolation film are recessed through the etching. The photosensitive film pattern is removed using the etching mask, and the remaining antireflective film is then removed.
Thereafter, the resultant substrate is subjected to a light etch treatment (LET) under conditions of low voltage and in the atmosphere containing a large quantity of O2 gas, thereby removing a damaged layer and a carbon pollutant formed on a surface of the substrate when the substrate is recessed.
Thereafter, a gate is formed on a stepped portion of the active region and the etched isolation film, and the substrate is subjected to source/drain ion implantation to form the STAR cell.
However, the conventional method of manufacturing the semiconductor device to form the STAR cell has the following problems.
As described above, immediately after forming the recess through etching, the substrate is subjected to a light etch treatment in order to remove the damaged layer and the carbon pollutant formed on a surface of the substrate. Since the treatment entails etching of the substrate, profile deformation is caused at a top portion of the recessed substrate, thereby varying the characteristic of the device.
If the substrate is not subjected to the light etch treatment so as to prevent the profile deformation, the characteristic of the gate oxide film to be formed during a subsequent process may be deteriorated.
FIGS. 1A and 1B are photomicrographs illustrating profiles of an active region before and after being subjected to the conventional light etch treatment.
As shown in FIGS. 1A, an edge of the active region has a round profile before performing a dry etching. It is noted that a stable device characteristic is obtained in the manufactured device.
Meanwhile, referring to FIG. 1B, the edge of the active region has a vertical profile, after performing the dry etching. It is noted that a device characteristic, such as an electric field concentrated onto the edge of the active region in the manufactured device, is deteriorated.
In addition, the light etch treatment causes the profile deformation of the recessed top portion in the cell region, and loss of the active region in the peripheral circuit. Therefore, since a process condition is additionally adjusted, it is difficult to secure a process margin.